1. Field of the Invention
The present invention relates generally to a data storage system employing a redundant array of disk drives (RAID), and more particularly to a continuous media or video file server employing RAID techniques.
2. Background Art
In a data storage system employing RAID techniques, a set of disk drives are configured into a RAID set storing data and parity computed across the disk drives in the RAID set. Therefore, if one of the disk drives in the RAID set fails, data in the failed disk drive can be reconstructed from the data and parity information in the other disk drives in the RAID set.
In a file server employing RAID techniques, it is conventional to employ a file directory for mapping a file name to a list of data blocks stored in respective ones of the disk drives in the RAID set. The data blocks are typically of a fixed size, such as 512 bytes, that are conveniently handled by the buffers and control circuitry provided by commodity 5 and 1/4 inch or 3 and 1/2 inch disk drives. A write access to a data block involves reading old data from an addressed disk drive, reading old parity corresponding to the old data from another disk drive in the raid set, computing a change in parity by an exclusive-OR of the old data with the new data to be written, computing the new parity by an exclusive-OR of the change in parity with the old parity, and then, in a single transaction, writing the new data and the new parity back to the respective disk drives.
It is desirable to use a data storage system employing RAID techniques for storing continuous media such as digital video. This could be done by using the conventional RAID techniques described above for a file server. However, this has been found to cause computational inefficiencies and load balancing and data availability problems under normal operation and under a failure mode of operation when there has been a failure of one of the disk drives in the RAID set.